Expression of type 1 angiotensin II receptor subtypes and angiotensin II-induced calcium mobilization along the rat nephron

Abstract

The localization of two type 1 angiotensin II receptor subtype mRNA, AT(1A) and AT(1B), was determined by reverse transcription-PCR on microdissected glomeruli and nephron segments. The coupling sensitivity of these two receptor subtypes was evaluated by measuring variations in intracellular calcium ([Ca2+](i)) elicited by angiotensin II (Ang II) in structures expressing either AT(1A) or AT(1B) mRNA, using Fura-2 fluorescence. The highest expression of AT1 mRNA was found in glomerulus, proximal tubule, and thick ascending limb. In glomerulus, AT(1A) and AT(1B) mRNA were similarly expressed, whereas in all nephron segments AT(1A) mRNA expression was dominant (approximately 84%). The increase in [Ca2+](i) elicited by 10-7 mol/L Ang II was highest in proximal segments (Δ [Ca2+](i) is approximately equivalent to 300 to 400 nmol/L) and thick ascending limb (Δ [Ca2+](i) is approximately equivalent to 200 nmol/L). In glomerulus and collecting duct, the response was lower (Δ < 100 nmol/L). The median effective concentrations for Ang II were of the same order of magnitude in glomerulus (12.2 nmol/L), in which both AT(1A) and AT(1B) are expressed, and in cortical thick ascending limb (10.3 nmol/L), in which AT(1A) is almost exclusively expressed. The Ang II-induced calcium responses were totally abolished by the AT1 receptor antagonist losartan (1 μmol/L) but not by the AT2 antagonist PD 123319 (1 μmol/L). In the absence of external Ca2+, the peak phase of the response induced by 10-7 mol/L Ang II was reduced and shortened, suggesting that a part of the [Ca2+](i) increase originated from the mobilization of the intracellular Ca2+ pool. In conclusion, these results demonstrate that in the rat kidney: (1) AT(1A) is the predominant AT1 receptor subtype expressed in the nephron segments, (2) glomerulus is the only structure with a relatively high AT(1B) mRNA content, and (3) AT(1A) and AT(1B) receptor subtypes do not differ in their efficiency for the activation of calcium second-messenger system.